The refractive-index profile of an optical fiber essentially determines its significant transmission characteristics such as light-collection efficiency, bandwidth and containment of luminous energy. If the index profile can be ascertained during manufacture of a fiber, as by means of measurements made on a preform, departures from prescribed values can be readily corrected. Naturally, such ascertainment would have to be carried out in a nondestructive manner to preserve the continuity of the manufacturing process.
In a paper presented at the Sixth European Conference on Optical Communications, held Sept. 16-19, 1980 at York, England, an apparatus for such nondestructive measurement of the refractive-index profile of an optical-fiber preform has been described by I. Sasaki, D. N. Payne, R. J. Mansfield and M. J. Adams of the Department of Electronics of Southampton University, Southampton, England. Such an apparatus, which is now commercially available under the designation P101 Preform Analyzer, utilizes a monochromatic beam of parallel light rays transluminating a preform in a plane transverse to its axis. The beam and the preform are relatively shifted within that plane in a succession of steps and the transluminating rays are focused in each position by a spherical lens onto a photodetector after traversing a rotating chopper which correlates the ray path with a predetermined reference position to determine the angle of refraction undergone by the beam. This known system, however, is rather complex and consequently expensive on account of its movable parts and the necessary synchronization between their motion and the instants of measurement.